If you blew up dual 12s, what makes you think you won't blow up a 15?
The most likely cause of speaker damage is simply running them too hard. If a speaker begins distorting (farting out), you have to back off. If you need more volume or a deeper tone, then you need more or different speakers.
Running too much wattage into a speaker is a less likely situation; most will be torn up long before they overheat. If, however, the speaker never farts out, you can overheat it. Power compression is when the speaker simply won't get any louder. You turn up the amp and the additional power just heats the speaker. Eventually you burn it up.
Tone, loudness, power compression and the "farting-out point" are all determined by cabinet design. Some do better than others.